The subject matter of the present invention is a method of preparing terephthalic acid dichloride and isophthalic acid dichloride by the partial hydrolysis of p-xylene and m-xylene, respectively, which are perchlorinated in the side chain, said partial hydrolysis being performed in the presence of Friedel-Crafts catalysts.
According to German Pat. No. 708,149 (IG-Farben 1936), terephthalic acid dichloride is formed by hydrolysis, by pouring water into 1,4-bis-(trichloromethyl)-benzene, heated at 125.degree.-140.degree. C, in the presence of iron (III) chloride or aluminum chloride.
Also known is the preparation of terephthalic or isophthalic acid dichloride by the reaction of 1,4- or 1,3-bis-(trichloromethyl)-benzene with the corresponding phthalic acids, with or without the aid of Friedel-Crafts catalysts, in accordance with German "Auslegeschrift" No. 1,196,636, among others, in which a mixture of the trichloride and terephthalic acid or isophthalic acid, heated to about 100.degree. to 130.degree. C, is introduced continuously into a reaction zone maintained at a temperature of approximately 270.degree. to 330.degree. C, or else the trichloride is poured into a melt of terephthalic or isophthalic acid dichloride, and the reaction mixture obtained in each case is processed.
The last-named method is disadvantageous, because one starts out with two starting substances, and, in the case of the continuous reaction, one must first separately combine the starting materials by melting them, adding the isophthalic acid or terephthalic acid in solid form, and then cooling and crushing them. Moreover, it is necessary without catalyst to operate at the very high temperatures of 270.degree. to 330.degree. C.
The first-named method of preparing isophthalic acid dichloride or terephthalic acid dichloride from only one starting substance, through the direct hydrolysis of the particular bis-(trichloromethyl)-benzene (hexachloroxylene), seems simple, but it entails the following difficulties:
The speed of the reaction of the hexachloroxylenes with water is substantially lower, under comparable conditions, than that of the reaction of the hexachloroxylenes with the phthalic acid in question. Consequently, in the case of a relatively large throughput, unreacted water escapes as water vapor with the hydrogen chloride and carries hexachloro-p-xylene and acid chloride with it. In the pipelines and stripping apparatus that follow, hydrolysis to phthalic acids takes place, resulting in deposits which are difficult to remove and in clogging. In addition, Friedel-Crafts catalysts, especially FeCl.sub.3, when used in the direct hydrolysis of hexachloroxylenes with the yielding of HCl, bring about the formation of condensation products which, when the detention time is long and the temperatures are high, amount to considerable quantities and greatly diminish the yield in the distillation. But of all the Friedel-Crafts catalysts it is precisely FeCl.sub.3 that has the greatest accelerating effect at relatively low temperatures.